Automatic gain control circuits for reducing amplitude variations



Dec. 8, 1953 R. H. STAFFORD AUTOMATIC GAIN CONTROL CIRCUITS FOR REDUCING AMPLITUDE VARIATIONS Filed Feb. 20, 1950 .EEI.

' INVEN TOR. RICHARD H.8TAFFOIZD Patented Dec. 8, 1953 AUTOMATIC GAIN CONTROL CIRCUITS FOR REDUCING AMPLITUDE VARIATIONS Richard Hariand Stafford, East Orange, N. J.

Application February 20, 1950, Serial No. 145,132

1 Claim. 1;

My present invention relates to an improved means for reducing amplitude variation in frequency and phase modulated waves, and more specifically to a very fast acting automatic gain control circuit for accomplishing this efiect.

An important object of this invention is to provide a means for removing or substantially reducing amplitude variations from angular velocity modulated carrier waves.

Another important object or" this invention is to accomplish thi reduction in amplitude variations by making use of an automatic gain control circuit having a time constant small enough to follow the variations occurring in the amplitude of the carrier wave envelope.

Further objects and advantages will appear from the following detailed description taken in connection with accompanying drawings and appended claim.

Fig. 1 shows a circuit embodying this invention and confined within one amplifier stage.

Fig. 2A shows an electrically equivalent circuit replacing the diode rectifier and the time constant means of this invention.

Fig. 2B shows a simplified version of Fig. 2A, simplified for use as a basis for mathematically deriving the time constant equation for this invention.

This invention can best be understood by studying the following circuit descriptions, in connection with the appropriate figures.

In Fig. l is shown a typical intermediate frequency amplifier housed in the pentode tube 4. The I. F. signal input is fed to control grid 6, in the normal manner, through I. F. transformer l, 2 and 3 being the tuning capacitors for the primary and secondary respectively.

The I. F. amplifier is self-biased by resistor Ill and I. F. bypas capacitor l l, to that bias giving the required transconductance for a normal amplifier in that circuit position. Tube 4, therefore, operates without its normal gain being impaired. In the plate circuit of the amplifier tube the I. F. output transformer 23, tuned by capacitors 24 and 25 acts in the manner normal to amplifiers of this type.

Between the screen I of the amplifier tube and the B plus supply triode I8 is inserted, with its cathode if connected to screen 1, and its plate 2i connected to B plus.

Fart of the output of the amplifier tube 4 is taken from the plate 9, through capacitor l2, and rectified by rectifier l3, the cathode of the rectifier being connected to capacitor l2, the plate to triode control grid 29. The rectifier of course need not be of the vacuum tube diode type, but may be of the semi-conductor type, such as germanium diodes or equivalents.

I. F. choke I6 is used to apply the I. F. signal across the rectifier l3. Resistor l! is the rectifier load as well as the triode control tube grid leak. The time constant of the circuit is governed by the internal impedance of rectifier l3 which may be regarded as a resistive element, resistor l'l, capacitor 22, capacitor 26, and the inter electrode capacitances of the triode and pentode.

The circuit operation is as follows: Part of the I. F. amplifier output is applied to rectifier l3, through a coupling means consisting of capacitor l2 and inductance 5. Other coupling means could have been used and still be within the meaning of this invention. The rectifier [3 has such a polarity as to negatively bias the control grid 28 of triode I8. As the signal amplitude increases the triode I8 is biased more negatively than before, which increases the D. 0. resistance of the triode between cathode and plate. This increase in resistance of the triode causes a greater voltage drop to appear across it, and consequently a lower screen voltage is available for the pentode amplifier. This reduces the transconductance and gain of the amplifier stage.

Vice versa a decrease in I. F. signal amplitude through the mechanism described above, causes the screen voltage of the amplifier tube 4 to increase.

For a pure frequency or phase modulated I. F. signal, the time constant without any amplitude variations existing can be set so that it is not fast enough to impair the normal gain of the amplifier, or to cause distortion in the I. F. waves. A pure frequency or phase modulated I. F. signal is rectified by rectifier l3, and the D. C. potential developed across resistor I? biases the control grid 29 of triode M3 by an amount which varies with the strength of the signal. As long as the signal strength remains constant the screen potential will be maintained at a constant value. and it can be said that the screen voltage has reached the equilibrium state. This equilibrium is disturbed whenever the signal strength changes, and seeks a new value of screen potential. This sort of action will be recognized as automatic gain control, and since the I. F. signal was assumed to be pure frequency or phase modulation, frequency modulation automatic gain control can be said to exist. By proper choice of values for capacitors 22 and 26, and resistor ll, screen grid '1 of tube i is given a time constant the amplitude variation is sinusoidal. If the time constant is fast enough, during that interval in which the amplitude 'of 'the I-. F. -carrier increases, the amplifier screen potential willdecrease simultaneously, clue to the automatic gain control action described above. If the gain control action is working properly the gain:of-:the"v=; amplifier will be decreased by such an amount... that the sinusoidal increase in the I. F. carrier wave envelope will be completely removed from the' -amplifier output. By the proper choice oivalues for-capacitors22 and 26, and resistor l7, screen grid 7 of tube is given a time constant whichfis fast enough to 'allo'w amplitude variationsdue 'to amplitude modul'ation'of the carrier waves to be removed or greatly reduced by the above mentioned automatic gain control action.

Viceversa; due tothe control action'describedabove a sinusoidal decrease in the I. F. carrier envelopwwill causethe-screen potential to in crease simultaneously, to remove the amplitude variations from the amplifier output.

During the complete cycle-of sinusoidal variation' of 'the amplitude of the =1. F. carrier wave,

then; the Scream potential varie inversely with Itcan be seen that this; so

was called-abo'vethe frequencymodulation automatic' gain control screen potential.

Refer;-now, to Fig." 2A inwhich is sho'wn'the equivalent-"electrical circuit of the diode recti-- fier: l3, inconjunction' with'time constant re-- sister: 1?, and'time constantcapacitors 22, 26;

Cgl i C t; and C ii. In"Fig. 2A C ii refers to the grid tocathode tube'capacitanceof the'triode I8 I control tube, Cgp refers to the grid to plate ca-' pacitance-ofthe control tube' [8, and C ,k refersto thecatho'de to screengrid capacitance of amplifier tube 4;

This equivalent circuitis reduced still further in .FigL -2B'; inwhich R represents the internal impedance of'diode rectifier I3, Ri-the diode load resistor and C2 the" sum of all capacitances across resistoril in Fig. 2A. It willbe'noted 'in Fig. 2A

thatzallcapacitors shown are ultimately in par-- allel with resistor I1.

Letxus resort totransient' analysis to determin the time constant of Fig; 23:

Themcharge. of .the capacitor at 5 time equal to zero, QCg,O is assumed equal to zero.

Substituting .for z in Equation 1 weget:

'. 'sudden application of 4 And from Equation 3:

i2=R1C d R+RI M+RRIC2 =E (6) And:

R1 2 11-R+ R1[1 6 The-time constant. comes from the term (R+R1), Ema,

and is the reciprocal of the exponential multiplier-of t, according to electrical transients theory: The time constant is, therefore,

RELC The shu'ntcapacitance C2,of Fig. 2 B;-'is respon sible" for retarding the-growth of the current-"1n-"-- the-ii branch, and causes the Rrii dropto la'gthe potential difleren When-the" switch "S i closed capacitance -C: charges slowly through resistorR and simultaneously retards the growth of current thIOUghflC- sistor R1- A reversal-in R. F.' -volta'ge'-is;equiva1entto shorting out the battery in Fig."-2B'.=-'- This-maybe shown by leaving the switch'S closed-andin stantaneously shortingout the battery. At-t'he instant of timein-which this is done the charge stored in capacitance Cr'starts-to dischargeslow I 1y through resistors R and 'Rrinparallel'rThe discharge time-constantis equal to:

which is the; same equation .as derived for the; charging time constant; 1

The sum 'of the charge and discharge time con-- stants is .therefore twicethat of the :oharging: time: constants The-rectified direct currentpotentiai from dio'deplate l5 follows the'variations' in amplitude of theFM" carrier wave applied to the diode input cathode. I4, .without being" reey tarded by the chargeand:discharge'time'constants involved' because: the time constant. thus developed is muchifasterin time than the rise, and fall of any amplitude'jvariations ,in'the FM carrier signal.- 2

Now that theitim'e constant means 'has been 1; establishedlet. us return to Fig..2A; The control of the time constant. depends'on the ize of resistor l1, and the sizesof capacitances.22;and 26, plus the. tube inter-electrode capacitances. It can be seenthat capacitances 221 and 26 are in series, therefore, their combined capacitanea-p can never be greater than the smaller capacitance value.

Let us now assume. some practical values of resistance. and capacitancev to. be used as. an example Let resistor ll'be 50,-000 ohms. Letcapacitor l 22 .-be-1,000 10- faradsm Let capacitor 26 be zerofaradss- Let the control tube be a 604. Let the amplifier [tube be.a 6AK5. 1i From the tubebooksz Cmwill be 1.6 X107 farads.-. Cgk Will be l.8 .10- faradsi, c kzw be. ligible compared to capacitor Resistor R, according to theory is approximately equal to one-half the diode load resistance, in this case 25,000 ohms.

2RR C 2X25,000X50,000X3.4X 10 (R+ R 25,000+ 50,000

t,=2 0.0568 10- seconds f= 8.3 megacycles If, now, resistor i1 is increased in value and capacitors 22 and 26 are given high values of capacitance, the time constant could be made very slow,

In practical cases capacitor 22 may be given that value of capacitance which is optimum as an I. F. screen bypass, and capacitor 28 may be chosen as the time constant controlling element in conjunction with the resistance value given to resistor 11. By proper choice of values for capacitors 22 and 26 and resistor I1, screen grid 1 of tube It is given a time constant means which is controllable.

What I claim is:

In combination with an amplifier having a cathode, an anode, a control grid, a screen grid, a suppressor grid, said cathode being connected through a self-bias resistor paralleled by a by-pass capacitor to ground potential, tuned input and output circuits, said tuned input 40 circuit connected to said control grid and said tuned output circuit connected to said anode, and angular velocity modulated carrier wave input signals, being further combined with a signal responsive diode rectifier, and a control tube having a cathode, a control grid, and an anode, said rectifier having its anode directly connected to the control grid of said control tube, and its cathode connected through a suitable coupling means to the anode of said amplifier and the cathode of said control tube, said coupling means being parallelled to the anode of said amplifier 5 by said tuned amplifier output circuit, said control tube being responsive to bias voltages developed from the output of said rectifier, said control tube being connected between a positive source potential and the amplifier screen grid, the anode of said control tube connected to a positive source potential, the cathode of said control tube connected to the screen grid of said amplifier tube, a resistor serving in common as diode rectifier load and control tube grid leak said resistor being connected between said cathode and control grid of said control tube, two capacitors connected in series across said resistor in such a manner that the connection between the two said capacitors is at ground potential, said resistor and capacitors forming in conjunction with the internal impedance of said diode rectifier and the inter-electrode capacitances of the amplifier tube and the control tube a time constant circuit, said coupling means including capacitive means for applying part of the amplifier output energy to the diode rectifier, the polarity of said diode rectifier being such that an increase in carrier wave amplitude causes acontrol tube bias increase, a control tube resistance increase, and a corresponding decrease in amplifier tube screen potential, and vice versa for a decrease in carrier wave amplitude, said action constituting automatic gain control by varying the transconductance of the amplifier tube, the diode load resistor and the two series capacitors having values selected to afiord a screen grid bias circuit for the amplifier tube having a time constant which is fast enough to allow amplitude variations due to amplitude modulations of the carrier waves to be removed or greatly reduced by the above mentioned automatic gain control action, yet not fast enough to distort or remove the pure angular velocity modulated carrier waves due to the above mentioned automatic gain control action.

RICHARD H. STAFFORD.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,916,129 Schade June 27, 1933 2,185,612 Trevor Jan. 2, 1940 2,428,039 Royden Sept. 30, 1947 2,466,229 Goldberg Apr. 5, 1949 

